Hardware is Windows 7 desktop. Airport express shows as active in the Airport Utility. Light is green on Airport express. Itunes will not connect although works ok on desktop. Shows unknow error -15000. This was working before the last Itunes update and I got it to work somehow after that but it is now not working again. I have booted computer, updated AE software and rebooted AE.

SOURCE: new wireless network - now my airport express and airtunes does not work

Hi I was searching this site for a solution to a problem I am having and came across your post. I had the same issue and what I did was reset my airport express to factory settings (detailed in manual) - this enabled the Airport extreme to find/pair with it.

SOURCE: airport express streams audio for a short period then quits

Make sure that your setup for the Express is set to "join the network" and not extend.
Also remember that with wireless technology there can be interference from microwaves, phones, etc.
If you are running programs that use a lot of bandwidth, this will cause the drop outs as well.

If you have a newer Express, another cure can be to use the 5GHz band for the broadcast channel.

SOURCE: Airport Extreme Bases Station not detectable with utility

This depends... Reaching them wirelessly may be unreliable for a number of reasons, so, I'll proceed on the assumption that you are attempting to test in the simplest lowest-variable situation:
First, make sure that you have the latest Airport Utility for your Mac.
Connect your Mac to the Airport via a single Category 5 (or 5e or 6) cable. Factory-reset the Airpot and then wake up the Mac. If the utility still can't discover your airport, it's probably time for a trip to your local Genius bar as you most likely have a hardware problem.
Lather, rinse, repeat for each of the two airports.

SOURCE: I can't connect apple airport expresses with a siemens 587 route

When doing this keep your password to 13 characters as different companies apply different defaults to make up the required 13

SOURCE: Airport utility cannot find airport express - ask for rescan

connect a ethernet cable to it. find it in the utility and reset it.